The present disclosure generally relates to liquid absorption systems and methods for removal of contaminants from gas streams such as flue gas.
Power plants may combust various fuels, such as coal, hydrocarbons, bio-mass, waste products, and the like, in boilers, for example, to generate steam and electricity. Exhaust streams (e.g., flue gas) of such combustion processes are often treated to neutralize or remove various compounds, such as carbon dioxide, sulfur oxides, nitrogen oxides, and particulate matter, prior to discharge of the flue gas to the environment. These downstream processes include, among others, post-combustion capture systems.
In post-combustion processes used for industrial separation of acidic components such as H2S, CO2, COS and/or mercaptans from a gas stream such as flue gas, natural gas, syngas or other gas streams mainly containing nitrogen, oxygen, hydrogen, carbon monoxide and/or methane, liquid solutions comprising amine compounds or aqueous ammonia solutions are commonly used as a wash solution. The acidic components are absorbed in the amine based wash solution in an absorption process. This process may be generally referred to as the main scrubbing process.
In the amine based absorption processes, water soluble contaminants such as ammonia, acetone, formaldehyde, other aldehydes, amine compounds, degradation products of amine compounds, nitrosamines, or combinations thereof can be formed and transferred to the gas stream. In some systems, these contaminants are subsequently removed along with the acidic components during regeneration. In other systems, these water soluble contaminants remain soluble in the water and are not removed. As a result, these water soluble contaminants can deleteriously increase in concentration and, in some cases, create a saturated solution, thereby forming precipitates that can lead to blockages within the conduits or may become exhausted through the stack.
In Prior Art FIG. 1, a typical liquid absorption system for removing carbon dioxide and contaminants from a gas stream generally includes an absorption unit 101 arranged to allow contact between the gas stream to be purified and one or more wash liquids. The absorption unit represented in FIG. 1 includes a CO2 absorption section 102 and a water wash section 103. In some systems, these sections are a packed bed column. Flue gas, from which CO2 and contaminants are to be removed, is fed to the absorption unit 101 via line 104.
In the CO2 absorption section 102, the flue gas is contacted with a first wash liquid comprising an amine compound, e.g., by bubbling the flue gas through the first wash liquid or by spraying the first wash liquid into the flue gas. The first wash liquid can be fed to the absorption unit via line 105. Exemplary amine compounds include, without limitation, monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropylamine (DIPA), and aminoethoxyethanol (diglycolamine), and combinations thereof. The amine based wash solution may further include a promoter and/or an inhibitor. The promoters are generally utilized to enhance the reaction kinetics involved in the capture of CO2. Exemplary promoters include an amine such as piperazine or enzymes such as carbonic anhydrase or its analogs. The promoters may be in the form of a solution or immobilized on solid or semisolid surfaces. Inhibitors are generally provided to minimize corrosion and solvent degradation. In the CO2 absorption section 102, CO2 from the flue gas is absorbed in the first wash liquid.
The flue gas depleted of CO2 then enters the water wash section 103 of the absorption unit 101, wherein the water wash section 103 is arranged to allow contact between the flue gas and a second wash liquid, which is generally water. The second wash liquid is fed to the absorption unit via line 106. In the water wash section 103, contaminants remaining in the flue gas when it leaves the CO2 absorption section are absorbed. The contaminants can include the water soluble volatile degradation products such as ammonia, formaldehyde, degradation products of amine, nitrosamines, combinations thereof, and the like. The flue gas, which is now depleted of CO2 and contaminants, leaves the absorption unit via line 107 and is typically discharged into the environment. Optionally, the treated flue gas depleted of CO2 and contaminants may undergo further processing, e.g., particulate removal (not shown), prior to being released to the environment.
Traditionally, the spent wash water from the water wash section is simply drained to the main CO2 removal step, where it mixes with the amine solution and leaves the absorption unit via line 108. The spent wash liquid (amine and water) may then be recycled via a regenerator unit 109, wherein contaminants and CO2 contained therein are thermally separated from the used wash liquid. The separated CO2 leaving the regenerator 109 may be compressed via line 110. For ease of transport, the CO2 may be compressed to the order of 100 atm.
In the regenerator 109, the amine content in the wash is recovered and because the temperature within the regenerator is elevated throughout (e.g., as high as 100 to 150° C.), any volatile degradation products can be stripped off from the amine-rich solution together with the acid gases, e.g., CO2, H2S, and the like. As a result, the regenerator produces a regenerator overhead product 114 that generally includes the acid gases, the volatile degradation products plus a major fraction of water vapor. Depending on the solubility and on the process design (e.g., temperatures, pressure, etc.) of the regenerator overhead condensing and post treatment system, these contaminants will either leave the system with the acid gas or are solved in the aqueous condensate generated in the condenser 112 or overhead system. As a result, the amount of contaminants can be kept relatively low. Also, since amine degradation is minimal in these types of system, the formation of volatile degradation products is equally low. However, in advanced amine based systems, which are generally adapted to the application for CO2 capture from low pressure flue gases, amine degradation is significantly higher leading to the production of higher amounts of volatile degradation products. Moreover, as will be discussed below, the absorption and regeneration units are of a significantly different design, which affects handling of the volatile degradation products.
FIG. 2 schematically illustrates a prior art amine based system for treating low pressure flue gas. Generally, a high water consumption and/or production of an aqueous emission are not desirable in a post combustion system. As such, the design of these advanced systems is to provide a neutral water balance, meaning that the amount of water vapor entering the plant via the flue gas is the same amount of water vapor leaving the water wash with the stack gas stack such that the water inventory of the whole system stays constant. In such as design, the wash water utilized in the water wash is generated self sufficient by condensing a part of the water vapor contained in the treated gas as coming from the main CO2 absorption.
The system generally includes an absorption unit 201 arranged to allow contact between a gas stream to be purified and one or more wash liquids. The absorption unit comprises an amine wash section 202 for CO2 absorption and a water wash section 203 for contaminant removal. Intermediate sections 202 and 203 is a condenser 212.
Flue gas from which CO2 is to be removed is first fed to the absorption unit 201 via line 204. In section 202, the flue gas is contacted with a wash liquid comprising an amine compound, e.g., by bubbling the flue gas through said wash liquid or by spraying the wash liquid into the flue gas. The amine wash liquid is fed to the absorption unit via line 205. CO2 from the flue gas is absorbed in the amine wash liquid. Flue gas depleted of CO2 in the CO2 absorption section 202 then enters the water wash section 203, wherein the flue gas contacts a second wash liquid, which is generally water, for removing contaminants from the flue gas. The second wash liquid is fed to the absorption unit via line 216.
As shown, the wash water utilized in wash water section 203 is generated self sufficient by condensing part of the water vapor contained in the treated gas coming from the CO2 absorption section 202. Excess water is not discharged as an effluent but instead is sent to the amine wash solution loop via line 214. Flue gas depleted of CO2 and contaminants leaves the absorption unit via line 207. The used first and second wash liquids containing absorbed CO2 and contaminants leave the absorption unit via line 208.
The used first and second wash liquids may be recycled via a regenerator unit 209, wherein the acid gases such as CO2 are separated from the wash liquids. A portion of the used wash liquids can be heated via heat exchanger 215 and fed to a mid-section of the regenerator (e.g., typically 100 to 150° C.) via line 218 or fed to the top portion of the regenerator unit 209 via line 220, which is at a markedly lower temperature so as to minimize the energy losses due to the latent heat of the water vapor (e.g., typically 40 to 60° C.). The regenerated wash solution is withdrawn from the lower section 222 and provided to a reboiler 230 positioned downstream of the regenerator and arranged to receive the regenerated wash solution. The separated CO2 leaves the system via line 210.
The reboiler 230 boils the regenerated wash solution to form steam 232 and a hot regenerated wash solution (amine lean) 234. The hot regenerated wash solution 234 is provided to the absorber unit 201 for removal of gaseous contaminants from the gas stream 204. The hot regenerated solution 234 may be provided directly to the absorber unit 201 for re-use. However, to take advantage of the thermal energy present, the hot regenerated wash solution is provided to heat exchanger 215, where it exchanges heat with the used wash solution 205.
In such designs, because of the temperature profile within the regenerator (i.e., cooler at the top portion, hotter at the bottom portion so as to reduce energy consumption), the volatile degradation products will not leave the regeneration system with the CO2 but rather remain dissolved in the amine solutions where they could build up to undesirably high concentrations. As a result, the volatile degradation products accumulate during cycling between the amine solution and the wash water. The volatile degradation products can then undesirably break through the stack or form solid depositions within the conduits of the system, e.g., ammonia may form ammonium bicarbonate or carbonate. As such, the newer systems are prone to the wash water failing to achieve the desired removal of the water soluble volatile degradation products or that blockages may occur due to buildup of solid depositions resulting from the accumulation and saturation of the volatile degradation products.
Accordingly, there is a need for volatile degradation product removal from amine plant wash water especially as it relates to designs where the only removal of the degradation products are through volatilization and discharge with the CO2.